Various protocols have been developed to allow high-speed communication between nodes of a network. One such example is Gigabit Ethernet, which is specified in IEEE 802.3-2005. In a Gigabit Ethernet network, nodes communicate with one another using frames whose individual bits are expected to be produced at a nominal rate of 10 bits per second and with a line signaling rate (physical layer) of 1.25×109 bits per second. Therefore, the nodes of the network comprise oscillators (clocks) that are expected to operate at a nominal frequency of the signaling rate of 1.25×109 Hz (=1.25 GHz), although a certain tolerance is permitted, in order to facilitate the design of compliant components and reduce their cost. Specifically, the components in a node that is compliant with the IEEE 802.3-2005 standard are designed to tolerate clocks operating at a frequency within the range of +/−125 MHz (i.e., +/−100 parts-per-million (ppm)).
Because the clocks in different nodes run independently of one another, the actual frequency of a clock operating at a given node may differ slightly from one node to the next. For example, the worst-case absolute disparity in the actual frequencies of the clocks operating in two nodes compliant with the IEEE 802.3-2005 standard can be as high as 200 ppm. For high link utilization (close to 100%), the result over time of this data rate imbalance is known as a “sync slip”, which is created between the data traveling in one direction and the data traveling in the opposite direction between the two nodes. Under a sustained traffic load at the data layer, such sync slips (also known as buffer underflow and overflow) will happen at regular time intervals; in fact, the higher the link utilization, and the larger the frame size, the more frequent the occurrence of sync slips per unit time. Sync slips have an impact not only on legacy data services such as fax and voice communication, but also on newer data traffic profiles such as IPTV, VoD and Circuit Emulation, as these are typically traffic profiles that can be on the order of hours and carry time-sensitive and loss-sensitive information.
To cope with sync slips, the nodes of a frame-based network (such as Gigabit Ethernet) can rely on the use of an inter-frame gap (IFG), which is effectively a digital “padding” between successive frames being transmitted over a given point-to-point link. By manipulating the size of the IFG to account for mismatches between the actual frequencies of the clocks operating in the nodes at either end of the point-to-point link, sync slips can be compensated for without impacting the integrity of the frames and, as a result, the data carried therein.
However, the use of inter-frame gaps prevents maximal exploitation of the link's capacity to carry data in a point-to-point environment. Specifically, the use of inter-frame gaps trades off part of the link capacity against the provision of a margin for handling sync slips. Further, under-exploitation of the link capacity is inevitable when the clock at one or both of the nodes communicating via the link operates at an actual frequency below the nominal frequency specified by the prevailing communication standard (albeit still within the tolerances set by the standard). In such cases, the actual throughput of point-to-point bidirectional communication will be governed by the lower of the two actual frequencies.
Thus, there is a need in the industry to improve the utilization of a point to point communication link between network elements that clock data to and from the link at high speeds.